The present invention relates to a method and device for determining the internal pressure in an eye, the so-called intraocular pressure (IOP).
As long-term increased pressure in the human eye can lead to blindness, the pressure is routinely measured at all eye clinics. An applanation method is normally used at the clinic, e.g. the so-called Goldman applanation tonometer, which means that a probe is brought to press against the eye until a predetermined deformation is reached and the force required is read.
The basis of the pressure determination is then the known relation between pressure, force and area:
P=F/A where P=pressure, F=force and A=area
The internal pressure of the eye can thus be calculated from the contact force against the eye and the area of deformation of the eye.
To establish that a specified deformation (area) has been reached, a fluorescent chemical is introduced to the eye and the eye is illuminated so that changes in light reflection at deformation can be read.
Another method is used when the demand on accuracy is not so high. It is a method where a jet of pressurised air is used to deform the eye with a specified force, whereby the deformation is read by detecting light reflections. This method has no physical contact between a fixed object and the eye.
Both methods are based on a force deforming the eye, which the patient can experience as being uncomfortable or painful, even though local anaesthetic is used with, for example, the Goldman method.
In addition, the former method has shown to be sensitive to astigmatism, as light refraction is always employed during measurement of the deformation of the eye. The latter method has documented shortcomings in precision and is thus not used when the nominal pressure is to be determined, but is often used instead by opticians, etc., for an initial measurement of the magnitude of the pressure.
There is always a risk of damaging the eye, especially the cornea, when the eye is pressed. This is one reason why it is desirable to minimise the contact force against the eye. The lowest force possible is determined according to the equation above by the area or deformation that is needed for this area to be correctly detected. The light reflection method that is used to detect or read the area of deformation requires a relatively large area for a correct reading and thus an equivalent relatively large force.
It is the aim of the present invention to alleviate or overcome the disadvantages stated above for known methods and devices for measuring the internal pressure in an eye.
This aim is achieved with a method and device that is first mentioned above and that has the characteristics that are defined in the following independent claims.